1. Field of the Invention
The present invention relates to sealing elements for sealing between relatively movable mechanical components, and more particularly relates to rotary sealing elements for use in high temperature applications, and a method of making same.
2. Description of the Related Art
a. Prior Art O-Ring Energized Lip Seals.
The Parker Hannifin Corporation manufactures a line of well-known O-ring energized two piece lip seals under the registered trademark “POLYPAK,” as shown in their Product Bulletin No. 5205B-1 titled “LC Profile Seal.” The POLYPAK seal is comprised of an O-ring and a lip seal shell. The O-ring is used as an expander ring that is compressed during installation to load the lips. The O-ring is often a different material from the lip seal shell, with each material selectable based on performance characteristics. In dynamic service, the O-ring is sometimes prone to dislodgement that allows leakage, as disclosed in Parker Hannifin Product Bulletin No. 5205B-1. O-ring dislodgement can also lead to contaminant ingestion and accelerated seal wear.
b. Prior Art Spring Energized Lip Seals
Many companies manufacture spring energized two piece lip seals for dynamic service, where a spring energizes the dynamic lip against a relatively movable surface of a machine component. Some examples of types of springs that are used include cantilever springs, garter springs, and canted coil springs. When such seals are used to exclude heavily contaminated environments, the springs are subject to dislodgement, causing leakage, contaminant ingestion, and accelerated seal wear. Such seals are unsuitable for applications where the differential pressure may occur in either direction. Such seals can only hold pressure when the differential pressure is acting from the side of the seal where the springs are located, so that the pressure acts interior of the dynamic lip to force the dynamic lip tighter against the relatively movable surface. When differential pressure occurs in the opposite direction, the pressure lifts the seal away from the relatively movable surface, allowing fluid to blow past the lip.
c. Prior Art Hydrodynamic Rotary Seals
Kalsi Engineering, Inc. markets a line of well-known patented dynamic seals under the registered trademark “KALSI SEALS.” These seals, which are hydrodynamically lubricated seals of one-piece construction, are widely used in oil well drilling and in other rotary equipment. These seals exploit the principles of hydrodynamic lubrication and contaminant exclusion that are set forth in U.S. Pat. Nos. 4,610,319; 5,230,520; 5,678,829; 5,738,358; 6,109,618; 6,120,036; 6,315,302; and 6,382,634. These patents are incorporated herein by reference for all purposes. The basic principle of hydrodynamic lubrication and contaminant exclusion of the KALSI SEALS brand seal product line is given in U.S. Pat. No. 4,610,319. U.S. Pat. No. 5,230,520 discloses a static lip that provides reduced seal distortion, improved seal stability and improved abrasive exclusion. U.S. Pat. No. 5,678,829 discloses an environmental side groove that can be used to reduce interfacial contact pressure to promote better lubrication and allow higher speeds. U.S. Pat. No. 5,738,358 discloses a dual modulus construction that can be used to reduce interfacial contact pressure to promote better lubrication and allow higher speeds and higher differential pressures. U.S. Pat. No. 6,109,618 discloses an aggressive hydrodynamic geometry that lubricates better under adverse conditions. U.S. Pat. No. 6,120,036 discloses a chamfered lip construction that can be used to control interfacial contact pressure, and that resists extrusion damage. U.S. Pat. No. 6,315,302 discloses an axially constrained geometry that provides improved abrasive exclusion in applications having little or no differential pressure, or low levels of reversing differential pressure. U.S. Pat. No. 6,382,634 discloses a geometry that provides additional sacrificial material to compensate for abrasive wear.
The rotary seals that are marketed by Kalsi Engineering are installed with radial interference (i.e. compression), and seal by blocking the leak path. Ultimate life is governed by the ability of the seal to remain resilient and maintain adequate interfacial contact pressure (i.e. to maintain sufficient contact force against the mating counter-surface of the shaft). Loss of resiliency is called compression set. Compression set is a measurement of the lack of rebound after a specified period of compression, and is usually expressed as a percentage of the original compression. As with most forms of elastomer degradation, compression set is worse at higher temperatures, and is influenced by media compatibility.
The single material, homogeneous elastomeric rotary seals manufactured according to FIGS. 1, 2, 2A, 3 and 5 of U.S. Pat. No. 5,230,520 are typically constructed of highly saturated nitrile elastomer, also known as HSN or HNBR, but can also be made from FKM (fluorocarbon rubber) or TFE/P (Tetrafluoroethylene and Propylene Copolymer). Testing has shown that when such seals are constructed from HSN, they are only appropriate for temperatures of up to about 300° F. Beyond that temperature, hardening and accelerated compression set significantly limits effective life. Such seals are widely used in downhole oil well drilling equipment. As oil wells get deeper, the need for an economical rotary seal that can withstand temperatures in the range of 300° to 400° F. has become critical. Increasingly higher rotary speeds are also being used for oil well drilling, which drives up the actual seal temperature. Conventional FKM elastomers have good compression set resistance and chemical resistance at high temperature, but are unsuitable for use in an oilfield rotary seal of the type shown in FIGS. 1, 2, 2A, 3 and 5 of U.S. Pat. No. 5,230,520 because of severe cracking of the dynamic interface that occurs at temperatures as low as 300° F., and because conventional FKM has poor abrasion resistance characteristics that make it unsuitable for sealing high differential pressures. FKM is often referred to as “VITON,” which is a trademark of DuPont Performance Elastomers. The elastomer TFE/P (commonly referred to by the Asahi Glass Co., Ltd. trademark “AFLAS”) is a fluorocarbon polymer that has good high temperature dynamic properties and good chemical resistance, but is not well suited for use as a single material downhole rotary seal of the type shown in FIGS. 1, 2, 2A, 3 and 5 of U.S. Pat. No. 5,230,520 due to its extremely poor compression set resistance characteristics. Compared to FKM, TFE/P has substantially less high temperature compression set resistance (i.e. TFE/P has higher compression set than FKM in high temperature conditions).
HSN dual modulus seals constructed in accordance with FIG. 9 of U.S. Pat. No. 5,738,358 provide a lower lip loading than those of U.S. Pat. No. 5,230,520, and therefore allow cooler operation at higher rotary speeds. The dual modulus construction is also more resistant to lubricant pressure-induced extrusion damage. In low differential pressure conditions, or in conditions of low level reversing differential pressure, the contact pressure distribution of such dual modulus seals isn't suitable for abrasive exclusion unless the chamfered lip construction of U.S. Pat. No. 6,120,036 is employed. In high differential pressure conditions, the lubricant pressure-induced flattening of the chamfered lip construction may compromise abrasive exclusion. As used herein, “modulus” or “elastic modulus” of an elastomer can be estimated in accordance with FIG. 1 of ASTM D 1415-83, Standard Test Method For Rubber Property—International Hardness.
HSN dual modulus seals constructed in accordance with FIG. 9 of U.S. Pat. No. 5,738,358 are very expensive to produce, which has limited their widespread use. The best production method has proven to be a multi-step method, which is labor intensive. The higher modulus inner part, which is referred to as the “insert” is first molded and partially cured (i.e. partially vulcanized), then cooled, trimmed and inspected. After trimming, several proprietary labor-intensive processes are used to ensure that the insert will effectively bond to the lower modulus energizing material. During the final molding, the partially cured insert is manually inserted into the hot mold cavity, and rotated to achieve proper timing between the wavy shape of the mold and the wavy shape of the insert. Only then can the lower modulus material be introduced into the mold cavity for final molding and curing. After this, the finished part must be re-cooled, re-trimmed and re-inspected. Various kinds of defects can occur at nearly every manufacturing step, and as a result the rejection rate is often high. The manufacturing process is not considered desirable for use with very thin inserts.
Several attempts have been made to produce dual modulus seals using two different peroxide cure-based HSN materials in a one step molding process, where the two uncured materials are molded simultaneously together in the mold cavity. The uncured higher modulus material doesn't always stay in place, and becomes dislodged and/or distorted due to the molding pressure and the movement of the uncured lower modulus material during the molding process. Even worse, the higher modulus material doesn't bond well to the lower modulus material, and could separate if ever used in actual service. In fact, the bond can be so poor that the two layers can sometimes be stripped apart by hand. In any case, the bond has been weaker than that of either of the two HSN materials.
It is desirable to have a rotary seal suitable for sealing between two members allowed to move relative to each other for use in a high temperature environment, including temperatures exceeding 300° F. It is further desirable that the rotary seal partition a fluid on one side of the seal from a second fluid on a second side of the seal. It is desirable that the rotary seal remain resilient and maintain adequate interfacial contact pressure with the mating counter-surface of the rotatable member.